When we came home from Phoenix a month ago, we saw some other commercial jets very close to us. First we met one, going opposite direction to us. Looked to be maybe 1/2 mile to 1 mile below us and even looked closer than that horizontally (although I suspect it was likely at least a mile away?). When you're both going about 500 mph and meeting, you don't get much of a look!

Then only about 1/2 hour later one crossed in front of and below us, similar distance. Man, that looks fast!